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Ad-hoc analysis

I often pull emails into a database to analyze them, but sometimes I want something simpler. Emails are typically stored in one of two ways: mbox format, where an entire mailbox is stored in a single file, and maildir format, where a mailbox is a directory with one file in it for each email.
My desktop mail application is Mail.app on OS X, and it stores messages in a maildir-ish format, so I’m going to work with that here. If you’re using mbox format mailboxes it’s a little trickier (but you can use a tool called formmail to split an mbox style format into a maildir directory and go from there).
I want to gather some statistics on mail I’ve sent to abuse desks, so the first thing I do is open up a terminal window and change directory to where my “Sent Messages” mailbox is:
cd Library/Mail/V2/IMAP-steve@misc.wordtothewise.com/Sent Messages.mbox
(Tab completion is really useful for navigating through the mailbox hierarchy.)
Then I need to go through every email (file) in that directory, for each file find the “To:” header and check to see if it was sent to an abuse desk. If it was sent to an abuse desk I want to find the email address for each one, count how many times I see that email address and find the top twenty or so abuse desks I send reports to. I can do all that with a single command line:
find . -type f -exec egrep -m1 '^To:' {} ; | egrep -o 'abuse@[a-zA-Z0-9._-]+' | sort | uniq -c | sort -nr | head -20
(Enter that all as a single line, even though it’s wrapped into two here).
That’s a bit much to understand all at once, so lets redo that in several stages, with an intermediate file so we can see what’s going on.
find . -type f -exec egrep -m1 '^To:' {} ; >tolines.txt
The find command finds all the files in a directory and does something with them. In this case we start looking in the current directory (“.”), look just for files (“-type f”) and for each file we find we run that file through another command (“-exec egrep -m1 ‘^To:’ {} ;”) and write the result of that command to a file (“>tolines.txt”). The egrep command we run for each file goes through the file and prints out the first (“-m1”) line it finds that begins with “To:” (“‘^To:'”). If you run that and take a look at the file it creates you can see one line for each message, containing the “To:” header (or at least the first line of it).
The next thing to do is to go through that and pull out just the email addresses – and just the ones that are sent to abuse desks:
egrep -o 'abuse@[a-zA-Z0-9._-]+' tolines.txt
This uses egrep a second time, this time to look for lines that look like an email address (“‘abuse@[a-zA-Z0-9._-]+'”) and when it finds one print out just the part of the line that matched the pattern (“-o”).
Running that gives us one line of output for each email we’re interested in, containing the address it was sent to. Next we want to count how many times we see each one. There’s a command line idiom for that:
egrep -o 'abuse@[a-zA-Z0-9._-]+' tolines.txt | sort | uniq -c
This takes all the lines and sorts (“sort”, reasonably enough) them – so that identical lines will be next to each other – then counts runs of identical lines (“uniq -c”). We’re nearly there – the result of this is a count and an email address on each line. We just need to find the top 20:
egrep -o 'abuse@[a-zA-Z0-9._-]+' tolines.txt | sort | uniq -c | sort -nr | head -20
Each line begins with the count, so we can use sort again, this time telling it to sort by number, high to low (“sort -nr”). Finally, “head -20” will print just the first 20 lines of the result.
The final result is this:

It’s not just useful for mailboxes – you can use some of the same approaches to pull data and summary statistics out of raw log files too (“What different 4xx responses has this MTA seen recently?”).

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